There is a known method of projecting one image by allowing projection images projected from a plurality of projectors to partially or entirely overlap with each other. When brightness of the projection image is uniform, an overlapping portion of the projection images (also referred to as an overlap region) might have brightness being different from other regions. To cope with this, brightness correction needs to be performed for these overlap regions. Accordingly, methods for performing brightness correction of such overlap regions have been proposed (for example, refer to Patent Documents 1 to 3).
The methods described in Patent Documents 1 to 3, however, need manual calibration of brightness correction amount by a user, and the calibration needs complicated operation. Therefore, these methods are not suitable for use in the calibration after the start of system operation.